The performance of many devices, such as laser printers and optical memories, can be improved by the incorporation of multiple laser beams. For example, laser printers which use multiple beams can have higher printing speeds and/or better spot acuity than printers which use only a single beam.
In many applications, closely spaced laser beams of different colors (wavelengths) are desirable. For example, color printers which use closely spaced laser beams of different colors can overlap the beams, sweep those overlapping beams using a single raster output polygon scanner and a single set of optics, subsequently separate the individual beams using color selective filters, direct each beam onto a separate xerographic imaging station, develop a latent image for each color on a different recording medium, and produce a full color image by sequentially developing each latent image on a single recording medium.
One way to obtain closely spaced laser beams is to form multiple laser emission sites, or laser stripes, on a common substrate. While this enables very closely spaced beams, prior art monolithic laser arrays typically output laser beams at only one color.
However, various techniques are known in the prior art for producing different color laser beams from a monolithic laser array. For example, it is well known that a small amount of color difference can be obtained by varying the drive conditions at each lasing region. However, the easily achievable color difference is insufficient for most applications.
One method of achieving large wavelength separations is to grow a first set of active layers on a substrate to form a first lasing element which outputs light at one wavelength, and then to form a second set of active layers next to the first to form a second lasing element at a second wavelength. However, this method requires separate crystal growths for each lasing element, something which is not easily performed.
Another technique for obtaining different color laser beams from a monolithic laser array is to use stacked active regions. A stacked active region monolithic array is one in which a plurality of active regions are sandwiched between common cladding layers. Each active region is comprised of a thin volume that is contained within a laser stripe. The laser stripes contain different numbers of active regions that emit laser beams at different wavelengths. Several stacked active region structures are discussed in U.S. Pat. No. 5,157,680, entitled "Integrated Semiconductor Laser," issued 20 Oct. 1992 to Goto.
In a stacked active region monolithic laser array, current flows in series through the stacked active regions. The active region with the lowest bandgap energy will lase, thereby determining the color of the laser beam output from that part of the array. To provide another color output, the previously lowest bandgap energy active region is removed from part of the array and current is sent through the remaining stacked regions.
Stacked active region monolithic laser arrays can not only output closely spaced laser beams of different colors, but beneficially the output laser beams are axially aligned with each other (share the same optical axes). In practice, the stacked regions of a stacked active region monolithic laser array are very closely spaced; separations in the stack direction typically being about 100nm.
A big problem with stacked active region monolithic laser arrays is that they have been difficult to fabricate, particularly in the AlGaAs material system. This is at least partially because the proper stacked active regions must be formed in each part of the structure. Conceptually, this problem can be solved by simply growing planar epitaxial layers which contain the required active regions such that the bandgap energies of the active regions decrease as one moves towards the crystal surface. Then, one could simply remove active regions, as required, to obtain the desired wavelength from each region of the array. Finally, the required cladding layer and capping layers could be grown over the remaining active regions.
However, it is very difficult to precisely etch the areas between the active regions when those active regions are closely spaced. Further, because of undesired growths on many materials when those materials are exposed to air, such as oxide growths on some compositions of AlGaAs, it is very difficult to achieve the required high quality growths over the remaining active regions. Thus, the simple conceptual approach given above is difficult to implement in some material systems, for example those containing aluminum.
Therefore, it would be useful to have techniques for producing stacked active region structures capable of outputting closely spaced, multiple color laser beams in material systems which are subject to undesired oxidation upon exposure to the atmosphere prior to growth of overlayers.